1. Field of the Invention
The present invention relates generally to an image forming apparatus and a cut sheet conveyance control method capable of double-side printing using a color electrophotographic process, and more particularly to an image forming apparatus and a cut sheet conveyance control method for preventing a paper jam which may possibly occur upon fixation of back-side print following the front-side print.
2. Description of the Related Arts
With recent prevalence of color printers employing the electrophotographic process, demands are increasing for rapider printing operations, improved accuracies, diversified medium specifications, multi-functionality such as double-side printing, reduced size and lowered price. Among them, in the color image double-side printing, a cut sheet picked up from a cut sheet feed unit such as a cut sheet feed cassette is conveyed to a process unit for transfer of color images onto the cut sheet front side, which is then fixed by a fixing device for front-side color print and delivered to a cut sheet discharge path for standby. Then the cut sheet standing by on the cut sheet discharge path is switched back to a circulation path provided for the back-side printing and is reconveyed from the circulation path to the process unit for transfer of color images onto the cut sheet back side, after which it is fixed by the fixing device for back-side printing. The double-side printed cut sheet is finally discharged from the cut sheet discharge path into a stacker. The fixing device of such a color printer applies heat to the upper and lower heat rollers above and below the fixing device for rapid printing so that toners and cut sheets are simultaneously heated under pressure by the upper and lower heat rollers to thereby improve the fixing properties.
In the fixing device of the color electrophotographic printing machine, however, the paper jam upon the double-side printing will be attributed to the defective cut sheet releases from the heat rollers. In case of the color printing in particular, the cut sheet release property is originally poor as compared with the monochrome printing since the color prints require volumes of toners due to the overlapping transfer of different color toners, with the heat rollers being set to a higher temperature than the monochrome printer, and with the pressure nipping the cut sheet between the upper and lower heat rollers being higher than that of the monochrome printer so that the toners are thereafter fully melted by the fixing device for color development. Furthermore, in case of the double-side printing, upon the back-side print after the front-side print, the printed cut sheet front side having color images formed thereon is reheated and melted by the lower heat roller, resulting in a further impaired release property. Thus, the color printer for the double-side printing inevitably necessitates a supply of oil to the upper and lower heat rollers of the fixing device in order to improve the cut sheet release property to thereby obviate the possible paper jam. For this reason, different oil supply stations have separately been provided so far on the upper and lower heat rollers to supply oil over the roller surfaces upon the double-side printing. The oil supply stations include their respective oil-impregnated oil rollers provided correspondingly to the upper and lower heat rollers.
In the event of provision of the separate oil supply stations on the conventional upper and lower heat rollers, the separate oil rollers mounted on the upper and lower heat rollers may induce a complex, bulky and costly fixing device structure. The oil rollers are expendable supplies and need to be replaced with new ones when the number of prints reaches a predetermined service life count. However, the oil rollers impregnated with the same amount of oil may have different oil supply amounts, with the result that both the oil rollers have to be replaced with new ones for service life management when a paper jam occurs due to the insufficient oil residue of the oil roller on one hand, in spite of sufficient oil residue of the oil roller on the other. In addition, the upper and lower heat rollers of the fixing device are disposed above and below the cut sheet conveyance path to be nipped therebetween, so that the upper heat roller must be removed for the replacement of the oil roller mounted on the lower heat roller, which results in a complex structure for oil roller replacement and in a labor and time-consuming oil roller replacement work.
It is therefore the object of present invention to provide an image forming apparatus ensuring a stable supply of oil from a single oil supply station onto surfaces of upper and lower heat rollers of a fixing device to thereby prevent any possible paper jam upon the double-side color printing.
According to a first aspect of the present invention there is provided an image forming apparatus such as a color printer, capable of double-side printing, comprising a conveyance control unit which allows a cut sheet picked up from a cut sheet feed unit to be conveyed to a process unit for transfer of color images on a front side of the cut sheet, the front-side transferred color images being fixed by a fixing device, the cut sheet switched back to a circulation path after fixing being reconveyed to the process unit for transfer of color images on a back side of the cut sheet, after which the back-side transferred color images are fixed by the fixing device for discharge. In such an image forming apparatus capable of double-side printing of the present invention, the fixing device includes an upper heat roller and a lower heat roller which thermally fix images under pressure on a cut sheet passing therebetween, either the upper heat roller or the lower heat roller being provided with a single oil applicator which supply oil for cut sheet release. When conveying to the process unit a cut sheet B1 to be back-side printed delivered via the circulation path, the conveyance control unit forms a conveyance spacing between the cut sheet B1 to be back-side printed and a precedent cut sheet A5, the conveyance spacing conforming to one turn or more of the heat roller which is not provided with the oil applicator. In this manner, according to the present invention, when performing a back-side printing, after circulation, of the front-side printed cut sheets, the cut sheet conveyance is made with spacing between the adjacent cut sheets which conforms to at least one turn of the lower heat roller, so that the upper and lower heat rollers can run idle between the adjacent cut sheets, during which oil applied over the upper heat roller surface is applied over the entire periphery. For this reason, whenever the back-side transferred cut sheet enters the contact (nip) between the upper and lower heat rollers, oil is already uniformly applied over the upper and lower heat roller surfaces, thus securely preventing any possible paper jam attributable to the insufficient supply of oil to the lower heat roller. The fixing device is so structured as to allow oil impregnated into the oil roller to stably and evenly be supplied from the upper heat roller through the roller contact to the entire periphery of the lower heat roller. Thus, an extremely simple structure will suffice similar to that for the single-side printing, achieving a reduction in size and price of the fixing device for use in the double-side printing.
Herein, when the number of cut sheets i to be double-side printed is not more than the number of circulated cut sheets n which depends on the paper size and which is the maximum number of cut sheets permitted to lie on the conveyance path in the course of printing, the conveyance control unit provides a conveyance control to effect a continuous printing of back sides of cut sheets, following a continuous printing of front sides of cut sheets.
In case the number of cut sheets i to be double-side printed exceeds the number of circulated cut sheets n which depends on the paper size, the conveyance control unit provides:
I. a conveyance control to effect a continuous printing of the front-sides of the cut sheets till the n-th cut sheet of i cut sheets;
II. a conveyance control to alternate the back-side printing of (ixe2x88x92n)th cut sheet and the front-side printing of i-th cut sheet until the number of cut sheets i exceeds n and the number of remaining cut sheets reaches n; and
III. when the number of circulated cut sheets is last n, a conveyance control to effect a continuous back-side printing of the remaining cut sheets. The number of circulated cut sheets n is e.g., 4 for paper size A4. The number of circulated cut sheets n is e.g., 2 for paper size A3. The conveyance control unit provides a drive control of a registration roller which temporarily stops a cut sheet to be conveyed to the process unit, for skew correction, to thereby form a conveyance spacing between a cut sheet to be back-side printed and a precedent cut sheet, the conveyance spacing conforming to one turn or more of the lower heat roller. The conveyance control unit provides a drive control of a registration roller which temporarily stops a cut sheet to be conveyed to the process unit, for skew correction, to thereby form a conveyance spacing L1 between a cut sheet A5 to be front-side printed and a precedent cut sheet A4, the conveyance spacing L1 being shorter than the spacing which conforms to one turn or more of the lower heat roller. The fixing device comprises a fixing device body which encloses the upper heat roller and the lower heat roller; and an oil roller unit which encloses the oil roller, the oil roller unit being mountable on and dismountable from the fixing device body. The fixing device body includes a donor roller which intervenes between the oil roller and the upper heat roller, and the oil roller unit includes a cleaning roller which cleans the donor roller.
According to a second aspect of the present invention there is provided a cut sheet conveyance control method in which a cut sheet picked up from a cut sheet feed tray is conveyed to a process unit for transfer of color images on a front side of the cut sheet, the front-side transferred color images being fixed by a fixing device which includes an upper heat roller and a lower heat roller, the cut sheet switched back to a circulation path after fixing being reconveyed to the process unit for transfer of color images on a back side of the cut sheet, after which the back-side transferred color images are fixed by the fixing device for discharge. The cut sheet conveyance control method comprises the step of, when conveying to the process unit a cut sheet A4 to be back-side printed delivered via the circulation path, forming a conveyance spacing L2 between the cut sheet A4 to be back-side printed and a precedent cut sheet A5, the conveyance spacing L2 conforming to one turn or more of the oil-applicator-free heat roller of the fixing device. In case the number of cut sheets i to be double-side printed is not more than the predetermined number of circulated cut sheets n which depends on the paper size and which is the maximum number of cut sheets permitted to lie on the conveyance path in the course of printing, a conveyance control is provided to effect a continuous printing of back sides of cut sheets, following a continuous printing of front sides of cut sheets. In case the number of cut sheets i to be double-side printed exceeds the number of circulated cut sheets n which depends on the paper size, control includes:
I. a conveyance control to effect a continuous printing of the front-sides of the cut sheets till the n-th cut sheet of i cut sheets;
II. a conveyance control to alternate the back-side printing of (ixe2x88x92n)th cut sheet and the front-side printing of i-th cut sheet until the number of cut sheets i exceeds n and the number of remaining cut sheets reaches n; and
III. a conveyance control to effect, when the number of circulated cut sheets is last n, a continuous back-side printing of the remaining cut sheets.
The number of circulated cut sheets n is e.g., 4 for paper size A4. The number of circulated cut sheets n is e.g., 2 for paper size A3. The cut sheet conveyance method includes forming a conveyance spacing L2 between a cut sheet to be back-side printed and a precedent cut sheet, the conveyance spacing L2 conforming to one turn or more of the lower heat roller, through a drive control of a registration roller which temporarily stops a cut sheet to be conveyed to the process unit, for skew correction. The cut sheet conveyance method may include forming a conveyance spacing L1 between a cut sheet to be front-side printed and a precedent cut sheet, the conveyance spacing L1 being shorter than the spacing which conforms to one turn or more of the lower heat roller, through a registration roller which temporarily stops a cut sheet to be conveyed to the process unit, for skew correction.
The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.